One of the main problems with electrophotographic copiers arises from the desire for speed in copy production. After a photoconductor has been charged, the energy required to produce a latent image in light and shade of the original of sufficient contrast to produce an acceptable copy is a function of the quantum of light falling upon the photoconductor and the light sensitivity of the photoconductor. Ideally, the illumination of the photoconductor should be such that the brightest part of the image will be fully discharged while the darkest part of the image will leave the photoconductor fully charged. In practice, this is never achieved, owing to the limits of the light response of known photoconductors. In the current state of the art of photocopying machines, when the speed of producing copies exceeds about 30 copies per minute, the energy required to operate the copier approaches 1500 watts. Since the ordinary potential in office and house wiring is 110 volts, the power from a given outlet is limited to 1500 watts. Accordingly, to produce satisfactory copies at a higher rate, a special electrical installation will be required. This means that the copying machine cannot be decentralized, but must be located in the region of the higher voltage outlet. Furthermore, the high energy will produce thermal problems, both in respect of the photoconductor and in the environment, aside from the expense of energy consumption. Because of these problems, many efforts are being made to increase the light sensitivity of photoconductors.
1. Field of the Invention
My invention relates to a novel method of increasing the effective sensitivity of photoconductors, thus enabling me to increase the speed of electrophotographic reproduction of documents.
2. Description of the Prior Art
The following art is of interest in respect of or is referred to in this specification:
Steinhilper--U.S. Pat. No. 2,756,676 PA1 Schaefer et al--U.S. Pat. No. 3,892,481 PA1 Hayashi et al--U.S. Pat. No. 3,907,423 PA1 Brooke--U.S. Pat. No. 3,912,387 PA1 Brooke--U.S. Pat. No. 3,994,723
Steinhilper, which will be discussed more fully hereinafter, proposes to make multiple copies of an image produced from a single light exposure of an original. He recharges the photoconductor after each transfer of a developed image and enhances the recharged image by subjecting it to illumination. There is no teaching of increasing the speed of the xerographic reproduction process. The apparatus shown by Steinhilper has only one development station. There is no optical masking station. There is no showing of a biased toner applicator at a toning station where optical shielding is achieved.
Schaefer et al show an automatic control system for biasing a development electrode. This system can be used both for the mask-forming step, which is a salient feature of my invention, and for the development step as taught by Schaefer et al.
Hayashi et al show a reverse roller designed to remove excess liquid from the photoconductor after the latent image has been developed. I employ a roller of this type, insulated from ground and biased to a voltage of the same polarity as the charge on the photoconductor, but at a potential higher than the background potential, in order to ensure that no toner is deposited on the background areas of the image when the mask-forming step is performed.
Brooke U.S. Pat. No. 3,912,387 and its divisional U.S. Pat. No. 3,994,723 show detecting background areas which are underexposed and discharging them by light before development of the latent electrostatic image.